21. The elements carbon and silicon both form dioxides.
(a) Carbon dioxide is a simple covalent molecule but silicon dioxide has a giant covalent structure.
(i) Describe the covalent bond between a silicon atom and an oxygen atom in silicon dioxide, in terms of the particles involved.
(2)
ANSWER
The strong electrostatic attraction between the shared pair of electrons of the covalent bond and the nuclei of the silicon atom and the oxygen atom.
(ii) Compare and contrast the covalent bonding in carbon dioxide and silicon dioxide in terms of orbital overlap.
(3)
ANSWER
Similarities:
i. Both molecules contain a σ-bond involving head to head overlap
Difference:
i. Carbon dioxide has covalent bond with both sigma bond and pi bond which involves side to side overlap of p-orbitals.
(b) The shape of the carbon dioxide molecule affects its physical properties.
(i) Explain the shape of the carbon dioxide molecule.
(3)
ANSWER
There are two sets of bonding electrons and no lone pairs about the carbon atom leading to minimum repulsion and resulting in a linear shape / bond angle of 180°.
(ii) Explain the polarity of the carbon–oxygen bond.
(2)
ANSWER
The carbon atom is slightly positive δ+ and the oxygen atom slightly negative δ− because oxygen is more electronegative than carbon.
(iii) State whether or not the carbon dioxide molecule is polar.
Justify your answer.
(1)
ANSWER
The molecule is non-polar since the dipoles are in opposite direction so they cancel each other.
(c) The structure of silicon dioxide may be referred to as a diamond structure.
(i) Using your knowledge of the structure of diamond, suggest how the structure of silicon dioxide is similar to that of diamond.
(1)
ANSWER
Each silicon is bonded to four oxygen atoms in a tetrahedral arrangement.
(ii) Give a possible reason why silicon dioxide has a lower melting temperature than diamond, even though the Si-O bond is stronger than the C-C bond.
(1)
ANSWER
There are fewer bonds per atom in the structure of silicon dioxide so less energy is needed to break the bond.
